VoIP enables telephony over the Internet or through any other IP-based network. Many wireless networks such as UMTS (Universal Mobile Telecommunications System) networks currently support VoIP service for mobile devices. 3GPP LTE (Long Term Evolution) is a Third Generation Partnership Project that sets out to improve the UMTS mobile phone standard in order to cope with future requirements. So far 3GPP LTE assumes that fast link adaptation should be supported for VoIP. Fast link adaptation involves matching modulation, coding, and protocol parameters in accordance with conditions of the radio link.
In order to match the modulation and coding scheme, fast link adaptation involves quick channel state feedback to the transmitter. Unfortunately, this can introduce a substantial overhead, for example as high as 5 information bits/2 ms/user for full fast link adaptation during an HSDPA (High-Speed Downlink Packet Access) operation. The number of VoIP users can be very large. For example, it has been shown that about 300 voice users can be supported in 5 MHz, 12.2 KBPS AMR (Adaptive Multi-Rate) and 5% outage (see TR 25.814, Physical Layer Aspects for EUTRAN (evolved universal terrestrial radio access network)). If each VoIP user uses fast link adaptation, then the total overhead could be significant, especially on the uplink. This can reduce system capacity as well as increase link interference. Fast link adaptation using uplink signalling can also increase power consumption for mobile devices causing shorter battery life.
It has been shown that for low constant rate services like VoIP, most of the AMC (adaptive modulation and coding) gain comes from HARQ (Hybrid Automatic Repeat-reQuest) rather than from fast link adaptation. This is partially due to the fact that the variation of voice payload size is not large compared to that of background data. The effectiveness of fast link adaptation can be reduced for traffic featuring this low variation of payload size. For the most part, the HARQ process compensates for the fast-fading effect effectively.